The reason for this change was to install help files into an
unique path so that server package (which provides these help
files) is installable alongside mysql-libs (from CentOS, and it
provides those files).

This has been an issue (reported elsewhere) for a while wherein Percona's server
package conflicted with mysql-libs at file-level even though they were of
different category (mysql-libs shouldn't ideally provide those
help files).

Now, regarding fill_help_tables.sql itself, mysql_install_db is
the user of it (and probably the only one) and it works fine with
changes made to RPM packaging (the SHAREDIR is provided as
compilation argument to cmake) and mysql_install_db itself.